It’s always interesting when new evidence brings about a change to the prevailing scientific view of something. When I was a child, in the 1990s, I was interested in astronomy. And at the time, a lot of books talked about ‘Planet X’, and how Pluto alone could not explain the behaviour of objects in the outer solar system. As it turned out, by the early 1990s, Planet X was no longer necessary to explain Neptune and Pluto’s orbit. Around this time astronomers began to use computer imaging, and so began to discover a large population of objects beyond Pluto. These objects mostly fall into the Kuiper Belt, which stretches from 30 AU (the orbit of Neptune) out to about 50 AU (an AU is an astronomical unit – the distance from the Earth to the Sun; hence Neptune is 30 times further from the Sun than Earth). Pluto’s orbit is between 30 and 50 AU.

The Kuiper Belt is not the outer limits of our solar system, though. Beyond it there are more objects, collectively referred to as the Scattered Disk. These include (probably) Sedna, which was discovered in 2003, and Eris, discovered in 2005, which is heavier than Pluto and has a moon of its own. It was the discovery of Eris which led directly to the demotion of Pluto. Eris orbits the sun at between about 40 and 100 AU; Sedna between about 80 and 900 AU (it has a very elongated orbit).

Based on the behaviour of these Kuiper Belt and Scattered Disk objects, astronomers have now come out again in favour of the theory that there is a ninth planet in our solar system (see this news article, which includes a diagram showing the basis for the theory).

The proposed planet is a small ice giant (if that’s not a paradox) like Uranus or Neptune, maybe 10 times as heavy as Earth, orbiting the Sun in an eccentric orbit between 200 and 1200 AU. Based on its size and the fact that it has almost certainly cleared its neighbourhood, there would be no doubt it’s a planet. Telescopes have begun searching for it. If it is near the aphelion of its orbit (i.e. far from the Sun) it could be very hard to find.

What I find enjoyable about the whole thing is that it comes across a bit like an Agatha Christie novel:

The Kuiper Belt terminates suddenly at about 48 AU – why? Is there something big out there clearing the neighbourhood?

What dragged Sedna and the other sednoids into their current weird orbits?

Given that we only know about the Scattered Disk objects that are close to the perihelion of their orbits, what does probability lead us to expect is currently at their aphelion, and where are they? Or, per astronomer Mike Brown: “Sedna is about three-quarters the size of Pluto. If there are sixty objects three-quarters the size of Pluto [out there] then there are probably forty objects the size of Pluto ... If there are forty objects the size of Pluto, then there are probably ten that are twice the size of Pluto. There are probably three or four that are three times the size of Pluto, and the biggest of these objects ... is probably the size of Mars or the size of the Earth.”

It’s also interesting to think that the model of the Solar System we all grew up with (ie. Those who went to school between 1930 and 2005) is actually seriously incomplete. We think of the inner planets – asteroid belt – outer planets, but it may actually be more like inner planets – asteroid belt – middle planets – Kuiper belt – scattered disk – outer planet(s) – God-knows-what beyond that.

I don't really have anything to add to this, but I wanted to acknowledge that I too found the news of the probable existence of Planet IX (despite the fact that no one has actually seen it) interesting as well.

Though I still think that Pluto should be promoted back to being a planet and we should go back to talking about Planet X.

I, too, am finding it all fascinating. It's just hard to comprehend that there could be a big planet out there that has not been seen or discovered yet. It's a great reminder of how little we actually know still, even though we know so much.

It's very, very, very far away—if the closest it gets to the sun is 200 times farther from the sun than we are, that's 18.6 billion miles. The farthest point from the sun would be well over 100 billion miles out. And its "year" is 15,000 of our years, so it hasn't been at its nearest point to the sun in all of recorded human history.

And it's dark out there! Planets don't glow, so we can only see it by reflected light from the sun, which is very dim at that distance—at closest approach, less than a ten-thousandth as bright as sunlight on Earth. At the farthest range of its orbit, the sun's irradiance is less than a billionth what it is here. So it will be a tricky challenge to spot it.

This is pretty exciting stuff. And Túrin, your summary was very clear and concise. Have you considered blogging (or do you already)? With your book coming out, it would be a good way to connect with readers and build your "platform," which seems to be required these days. You have a big accumulation of worthy posts on this board that would let you get a good start without the need to focus on it daily.

_________________“There, peeping among the cloud-wrack above a dark tor high up in the mountains, Sam saw a white star twinkle for a while. The beauty of it smote his heart, as he looked up out of the forsaken land, and hope returned to him. For like a shaft, clear and cold, the thought pierced him that in the end the Shadow was only a small and passing thing: there was light and high beauty for ever beyond its reach.”― J.R.R. Tolkien, The Return of the King

Though I still think that Pluto should be promoted back to being a planet and we should go back to talking about Planet X.

It would actually become Planet XI, as promoting Pluto would mean promoting Eris as well. But yes, it was convenient when Planet X was actually Planet X.

Primula Baggins wrote:

It's very, very, very far away—if the closest it gets to the sun is 200 times farther from the sun than we are, that's 18.6 billion miles. The farthest point from the sun would be well over 100 billion miles out. And its "year" is 15,000 of our years, so it hasn't been at its nearest point to the sun in all of recorded human history.

And it's dark out there! Planets don't glow, so we can only see it by reflected light from the sun, which is very dim at that distance—at closest approach, less than a ten-thousandth as bright as sunlight on Earth. At the farthest range of its orbit, the sun's irradiance is less than a billionth what it is here. So it will be a tricky challenge to spot it.

This will be particularly true if it's dark (assuming it exists). Most of the distant objects that we've found are fairly reflective. Pluto is beige (something that we now know thanks to New Horizons!), Eris seems to be nearly white, and Sedna is red like Mars. If Planet 9 is an ice giant and is a darker blue than Neptune, it will be very hard to find.

ETA: And we've observed a black gas giant in transit across its star in another solar system, so it's not impossible.

Primula Baggins wrote:

This is pretty exciting stuff. And Túrin, your summary was very clear and concise. Have you considered blogging (or do you already)? With your book coming out, it would be a good way to connect with readers and build your "platform," which seems to be required these days. You have a big accumulation of worthy posts on this board that would let you get a good start without the need to focus on it daily.

I have considered it (and I've been a blogger in the past). I've found two issues, though - one is that my last site got hacked and taken down, so I'd need to sort out security, and the other is that most successful blogs I know and/or follow have a particular theme (and hence a readership) and I'm not sure what mine would be. My interests jump all over the place.

You could start close to your book (Australian history) and branch out quickly from there. And there are blog host sites that handle security for you to some extent. They give you a framework and a commenting system, and they seem to be able to handle major traffic as well as small, special-interest blogs (several major liberal political sites are still on Blogger).

_________________“There, peeping among the cloud-wrack above a dark tor high up in the mountains, Sam saw a white star twinkle for a while. The beauty of it smote his heart, as he looked up out of the forsaken land, and hope returned to him. For like a shaft, clear and cold, the thought pierced him that in the end the Shadow was only a small and passing thing: there was light and high beauty for ever beyond its reach.”― J.R.R. Tolkien, The Return of the King

That's a really good idea, Ax. I'm trying to figure out what I might do, assuming I do have new books coming out, and blogging on one topic doesn't interest me enough that I would be likely to be able to sustain it. But I could write about several topics in rotation (SF, planetary science, space exploration, and probably miscellaneous, which might include SF pop culture/TV/movies, a little bit of politics, and rambling).

I would probably harvest some of my less dumb posts from here to help out, as I suggested to Túrin. I wouldn't, of course, include anyone else's words or ideas, or any link to here (for me this is my home, my local, my refuge, and I intend to keep it separate from my fiction readership, assuming there is any).

Which makes me realize I couldn't use any Lasto posts, as those are public and somebody could theoretically (via Google) figure out that out that I'm Primula Baggins here. Ah well.

_________________“There, peeping among the cloud-wrack above a dark tor high up in the mountains, Sam saw a white star twinkle for a while. The beauty of it smote his heart, as he looked up out of the forsaken land, and hope returned to him. For like a shaft, clear and cold, the thought pierced him that in the end the Shadow was only a small and passing thing: there was light and high beauty for ever beyond its reach.”― J.R.R. Tolkien, The Return of the King

No, I can't/won't do that, Inanna. In my view anything I post here becomes part of this board (even if the words still belong to me). And I don't think posts should be removed even after discussion has moved on. This is a bit of a hot-button issue for me. I sometimes go back and read old discussions—sometimes to try to figure out how a problem happened, sometimes just for the fun of reading a lively discussion among intelligent people. Years and years ago someone left this board in anger and, as a parting shot, removed many of their own posts from the heart of some of our best early discussions. It left bleeding holes in those threads. It was that person's choice, but I will never think it was right.

However, if I think I made an interesting point, I can certainly write something new that makes it again. Prolixity, thy name is Prim.

ETA: I'm not talking about someone making a post they regret and deleting it soon after making it. That happens, and it's not the same issue at all. I'm not a fan of it, but I can't tell someone else that they must leave up a post that makes them uncomfortable; I would certainly rather see them edit as they feel they need to edit rather than, say, stop posting here.

This was different. Removing my own posts so I could use them elsewhere would also be different and in my view, not right.

_________________“There, peeping among the cloud-wrack above a dark tor high up in the mountains, Sam saw a white star twinkle for a while. The beauty of it smote his heart, as he looked up out of the forsaken land, and hope returned to him. For like a shaft, clear and cold, the thought pierced him that in the end the Shadow was only a small and passing thing: there was light and high beauty for ever beyond its reach.”― J.R.R. Tolkien, The Return of the King

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